We are evaluating the effects of oral contraceptives on the vascular system by studying the alterations in surface charge characteristics of the blood cells. When the surfaces of the blood vessels and blood cells become less negatively charged, conditions prone to thrombosis result. We have been able to show that contraceptive steroids cause a reduction in the net negative surface charge of blood vessel wall and blood cells: (1) The electrophoretic mobilities of platelets and red cells from women taking Ovral and Demulen were lower than the corresponding controls. There was also an alteration in coagulation behavior of these patients' plasmas. (2) Administration of mestranol lowered the surface charge of arteries and veins, as revealed by electroosmotic flow measurements of sections of canine aorta and vena cava. (3) Current induced occlusion times in rat mesentery vessels were markedly lowered upon administration of many contraceptive steroids. It is planned to study: (a) the effect of a few more (5) contraceptive steroid hormones on occlusion times in mesenteric vessels of rats (fed orally for 2 weeks); (b) study the alteration in surface charge of blood cells and blood vessel walls of dogs fed with these hormones. These results will be complementary to data obtained from rat mesenteric studies, and (c) correlate the changes in surface charge of platelets from women on contraceptives with the aggregation behavior.